1. Field of the Invention
The present invention relates to apparatus for monitoring a microcomputer circuit, and more particularly to such a circuit which can detect and indicate the execution, by the microprocessor, of an erronous program sequence.
2. Prior Art
Microprocessor circuits are sometimes subject to external influences, which can result in a functional disruption of the operation or a condition which is sometimes referred to as a system falling asleep. When such disruptions occur, nearly unlimited number of false program sequences can take place, which may result in reprogramming interface circuits or ports, or timing circuits, etc., and can also modify data, turn off interrupts, etc. Any of these conditions can drastically affect the operation of the microcomputer system and subsequent operations. In addition, dislocation of the program counter can lead to false program sequences in which the microcomputer enters the wrong sequence, or an endless loop of instructions.
It is desirable to recognize such functional disturbances and to automatically eliminate them as much as possible. Circuits designed for this purpose are referred to as "watchdog" circuits.
Malfunctions in microcomputer systems used for remote control purposes can be particularly critical. Remote control substations are frequently difficult to access, so that malfunctions must be avoided insofar as possible. Yet remote control devices are particularly subject to line disruptions, network fades of brief duration, or strong electromagnetic noise fields. These disruptions can have a number causes, such as, for example, lightning, switching transients in electrical systems or tracks, etc.
The present invention is of the type which incorporates an evaluation device connected to a check pulse channel which monitors the generation of check pulses and emits a control signal if the sequence of check pulses deviates from a prescribed anticipated sequence. When this occurs, the microprocessor is reset, or an alarm condition is activated which signifies a trouble report. Such apparatus is disclosed in general in the periodical Electronic Design, Volume 2, Jan. 18, 1977, pages 90 and 92. The arrangement described there comprises a resettable counter which is triggered at a regular interval, during normal operation of the microcomputer. When the trigger pulses arrive on time, the counter is reset by each pulse, and prevents it from counting up to its full radix, which would result in restarting the microcomputer system. Since the time required for the counter to count to its full radix is greater than the normal pulse repetition time of the check pulse, the counter does not reach its full radix as long as correct operation continues. The check pulses are generated by software, controlled by the microprocessor, and are made available through an output interface arrangement or a port line. The check pulse channel is connected to the microprocessor by a capacitor.
In the periodical Elektronik, Volume 23, No. 16, November 1984, pages 120 and 121, apparatus is disclosed for self monitoring a microcomputer circuit having a watchdog circuit, as well as apparatus for under-voltage monitoring. The microprocessor is reset when necessary, by reset signals produced by the watchdog circuit or by the under-voltage monitor apparatus.
It has been determined that apparatus such as that described above does not completely exclude the possibility of a microcomputer system skipping into a incorrect program sequence, in which the single monitored port is driven periodically, so that the watchdog circuit cannot respond. It is therefore desirable to provide an apparatus and method for providing more reliable operation.